In spectrophotometric analysis, estimates of the concentration of various analytes in a sample are determined by (a) transmitting light from one or more emitters through the sample, (b) measuring the amount of light transmitted through the sample from each emitter, and (c) using the measured light in a calibration equation to obtain the analyte estimates. To maximize the information obtained from the sample-under-test, each emitter typically emits light having distinct spectral content and center wavelength.
Typically, theoretical relationships along with statistical regression techniques are used to develop a calibration equation. In particular, a pre-specified equation form is chosen which relates the measured light to the analytes of interest. Constants present in this equation, called calibration coefficients, are then determined from a test collection of measured light vectors corresponding to known quantities of the analytes, as one skilled in the art will understand. However, to effectively utilize such a methodology the analytes must be distinguishable at the center wavelengths of light used. That is, the contribution of any one analyte to the measured light from all emitters cannot be a linear combination of the corresponding contributions of one or more other analytes. Moreover, no subcollection of the analytes should absorb light so strongly that absorption of light due to other analytes in the sample cannot be measured. Additionally, to cost-effectively provide such spectrophotometric analysis, there is an emphasis on minimizing the number of emitters used. It is well known, however, that there must be at least as many distinct emitters (each with its own unique spectral content) used as there are analytes whose concentrations are to be determined. Furthermore, in order to obtain accurate concentration estimates, additional emitters are often used, thereby creating a mathematically over-determined system. Thus, in conventional spectrophotometry, there is a cost versus accuracy tradeoff, wherein both cost and accuracy increase with an increase in the number of emitters used in such systems.
Accordingly, it would be advantageous to have a method and system for increasing the accuracy of such spectrophotometric analysis without incurring the cost associated with increasing the number of emitters. In photoplethysmography, maintaining a high degree of accuracy at a low cost is particularly important. That is, a high degree of accuracy is required in order to ensure patient safety and alert the clinician of critical situations, while cost containment pressures on the medical community necessitate low cost monitoring devices.